Modern wind turbines are commonly used to supply electricity into the electrical grid. Wind turbines of this kind generally comprise a tower and a rotor arranged on the tower. The rotor, which typically comprises a hub and a plurality of blades, is set into rotation under the influence of the wind on the blades. Said rotation generates a torque that is normally transmitted through a rotor shaft, either directly or through the use of a gearbox, to a generator. This way, the generator produces electricity which can be supplied to the electrical grid.
Wind turbines may comprise pitch systems that are employed for adapting the position of the blades to varying wind conditions by rotating each blade along its longitudinal axis. Pitch systems may be used in control of a wind turbine to e.g. maintain a certain rotor speed and/or limit loads on the blades. Determination of loads on the blades, and on the turbine in general, is considered to be an important task to be performed during operation of the wind turbine to confirm that the generated loads are, in fact, acceptable.
One of the most common ways of determining loads on a wind turbine is based on using load sensors arranged on the blades. These load sensors may generally be based on methods of detecting loads by using e.g. a pressure sensor, a throttle sensor or a strain sensor, and/or methods of detecting loads by using intensity modulated light within one or more optical fibres which may deform as the blade deforms. Load sensors may suffer failures or malfunctions due to e.g. freezing, mechanical wear and/or saturation to full scale or zero of the sensors.
The way the load sensors are distributed on the blades may also have its effect on the quality of load measurements. Loads on the blades and, in general, on the wind turbine may depend on many parameters and rather variable magnitudes of said parameters. Examples of some parameters playing a role in generating loads on the blades are e.g. wind speed, wind direction, rotor speed, pitch angles, rotor position, etc. Taking this into account, an ideal distribution of sensors does not exist. Some distributions will produce good measurements under some operational conditions and some other distributions will produce good measurements under some other operational conditions.
FIGS. 1a, 1b and 1c will be used to discuss some of these problems of prior art systems based on load sensors on the blades and aimed at determining loads on a wind turbine. For the sake of simplicity, only one force, namely the weight of the blade, will be considered in the following description.
FIG. 1a shows a frontal view of a common wind turbine comprising three blades 100, 101, 102, and a hub 110 carrying the blades 100, 101, 102. One of the blades 100 has several load sensors 105-107. FIG. 1b shows a lateral view of a region 108 of the wind turbine from a lateral point of view 109, in which the first blade 100 may have a determined pitch angle 112. And FIG. 1c shows a lateral view of the region 108 of the wind turbine from the same lateral point of vision 109, in which the first blade 100 has another pitch angle.
FIG. 1b and FIG. 1c show a first load sensor 106 and a second load sensor 107 that are positioned in a flap-wise axis of the blade 100. FIG. 1b and FIG. 1c also show a third load sensor 105 and a fourth load sensor 111 which are positioned in an edge-wise axis of the blade 100. In FIG. 1b, the blade 100 has a pitch angle 112 of ninety degrees, whereas in FIG. 1c, the blade 100 has a pitch angle of zero degrees.
At the pitch angle 112 of ninety degrees (FIG. 1b), loads due to the weight of the blade 100 are sensed by the flap-wise sensors 106, 107 with high accuracy, since the sensitivity of said sensors 106, 107 at this position is good for measuring these loads. However, the edge-wise sensors 105, 111 will not provide equally good measurements of loads caused by the weight of the blade 100, since the sensitivity of said sensors 105, 111 at this position is inappropriate to measure these loads.
Similarly, at the pitch angle of zero degrees (FIG. 1c), loads due to the weight of the blade 100 are sensed by the edge-wise sensors 105, 111 with high accuracy, since the sensitivity of said sensors 105, 111 at this position is good. However, the flap-wise sensors 106, 107 will not provide equally good measurements of loads due to the weight of the blade 100.
At some intermediate positions (pitch angles) with respect to the ones shown in FIGS. 1b and 1c, the edge-wise sensors 105, 111 can be more appropriate than the flap-wise sensors 106, 107 to measure loads caused by the weight of the blade 100, and vice versa. At some other intermediate positions (e.g. at a pitch angle of 45 degrees), neither the edge-wise sensors 105, 111 nor the flap-wise sensors 106, 107 may provide useful measurements of loads due to the weight of the blade 100.
These considerations about the weight of the blade can be extended, taking into account corresponding particularities, to other loads on the blades, such as e.g. forces derived from the wind, the rotation of the rotor, and so on. Absolute forces resulting from rather complex combinations of such forces may make a distribution of load sensors suitable to measure the resulting loads in only some operational conditions (faced during operation of the wind turbine), whereas in other conditions the load sensors do not function as well.
It may be conceived to provide a high number of sensors of different types that ideally can measure loads well in any circumstance. However, this option may be expensive and complex, since it is based on having many sensors and processing many signals provided by the sensors.
There still exists a need for a new method of operating a wind turbine which at least partially reduces the abovementioned problems. It is an object of the present invention to fulfil such a need.